The long-term goal of the work proposed here is to understand the production and change over time in patterns of motor behavior. The studies are informed by a dynamic systems perspective and expand the use of mathematical models that address coordination issues by focusing on the availability of dynamic resources from which humans select control strategies for solving movement problems. Recently such a model has been developed but limited in its validation to normal children and those with cerebral palsy (CP). This proposal addresses the generalizability of this modeling approach by focusing on a population with unique dynamic resources, Down syndrome (DS). It extends previous work by studying behavior across the lifespan in a population whose control problems are poorly understood. Focus is on walking because it emerges relatively early in life, is well practiced over time, complex, changes over the lifespan, is an important functional skill, yet has received limited directed attention by the scientific community. Specific aim #1 is to examine the capacity of the model to explain gait pattern differences in preadolescent children with DS and typically developing (TD) children as well as their ability to adapt to perturbations. Preadolescence is a time when performance on this task may be expected to be optimal. Comparison will be made between gait kinematics and the model's predicted dynamic gait strategies (i.e., relative stiffness and forcing functions. Specific aim #2 is to extend the application of this model across developmental levels, including new walkers and older walkers. At these levels optimality constraints may be quite different; they represent different levels of practice, as well as differences in the biomechanical and physiological constraints, within the same population. In particular, new walkers will be tested in a longitudinal study design in order to map the emergence of dynamic strategies within participants. Specific aim #3 is to concurrently validate the effectiveness of the model. This will include comparing the model's method for estimating stiffness and forcing to measures developed by other researchers, examining neuromuscular correlates of the model's estimates for stiffness and forcing, and determining if non-linear components in the model are required to minimize the error and maximize generalizability of the model. These studies have wide-ranging implications for a.) understanding the emergence of control strategies, b.) understanding general processes of change, and c.) impacting on clinical practice, especially physical and occupational therapy and orthopaedic medicine.